1. Field of the Invention
This invention relates to the field of measuring devices for determining the pitch of propeller blades and the like.
2. Description of the Prior Art
Several types of devices utilize bladed items such as propellers and fans. In general, the bladed items have two or more blades which extend radially from a central hub. A shaft normally is either received within an aperture defined by the hub or is made an integral part of the hub. The bladed item is rotated about the rotational axis defined by the longitudinal axis of the shaft.
The performance of the bladed item depends on many variables including the size, shape and pitch of the blades. The pitch refers to the angle between a blade and the rotational axis. A station for a blade is an area of the blade located a given distance from the rotational axis, which is also referred to herein as the base station line. The pitch of a blade may vary for the different blade stations. Bladed items must be carefully balanced with respect to the blade variables, and it is therefore desirable to have a means for measuring these blade variables.
Several devices exist for determining the pitch of blades. One such device is disclosed in U.S. Pat. No. 3,380,170, issued to Read on Apr. 30, 1968. The Read device includes a shaft on which a propeller or the like is received. A rectangular arm extends radially from the shaft and supports a second arm which extends downwardly and parallel to the shaft. A generally triangular pointer is pivotally attached to the second arm and is oriented in a plane normal to the first arm. When the two base points are positioned against the blade surface, the apex of the pointer indicates the degree of pitch on a protractor mounted appropriately on the second arm. The Read device is suitable for proximately determining the pitch of a blade for a given station. The Read device, however, does not permit measurement of the pitch of only a sector of the blade, a sector being defined as a portion less than a full station.
In U.S. Pat. No. 1,021,797, issued to Rice on Apr. 2, 1912, there is disclosed another pitch measuring device. The Rice device includes an arm which rotates about a pivot affixed to the propeller hub. The arm is supported away from the hub by an arcuate bridge member which includes a protractor scale. Slidingly received on the arm is a casing which supports a screw threaded measuring finger. To measure the pitch of a blade, the measuring finger is positioned against the blade at one end of the station or sector. The finger is then positioned against the blade at the other end of the station or sector by rotating the arm and the screw-threaded finger as required. Rotation of the finger results in displacement of the finger along a line parallel to the rotational axis of the arm. The rotation of the arm is determined by reference to the scale on the supporting bridge member. The finger casing is provided with a geared dial which reflects the axial displacement of the finger resulting from its rotation. The pitch may then be calculated from the rotational movement of the arm and the axial movement of the finger according to the known formula that pitch is equal to the axial movement multiplied by 360.degree. and divided by the rotational movement. The Rice device thus permits measurement of the pitch for any sector of the blade. The Rice device is, however, fairly cumbersome and does require relatively precise and expensive production of the measuring finger and associated, geared dial. A similar device using a movable measuring finger received by an arm attached at a fixed location on a shaft is disclosed in U.S. Pat. No. 2,717,453, issued to Wildt on Sept. 13, 1955.
The existing pitch measuring devices are suitable for particular applications. There are limitations, however, on the usefulness of these various devices. As previously noted, for example, not all existing devices are capable of measuring the pitch of each blade sector. Those capable of measuring the various sectors generally incorporate complicated or precision elements which add to the expense and potential inaccuracy of the devices. There is therefore a need for a pitch measuring device which is simple and inexpensive in construction and which may be used to easily and accurately determine the pitch of any blade sector. The measuring device also is most preferably of a design which may be readily adapted for use with the various sizes and shapes of bladed items.